Paper NS-TuP31
Enhanced Photovoltaic Performance of DSSC using Broccoli -Like TiO₂ Film with Surface Modification by Zinc Acetate Solution

Tuesday, November 10, 2009, 6:00 pm, Room Hall 3

The broccoli-like TiO₂ film as working electrode for dye-sensitized solar cell (DSSC) was fabricated successfully by flat-flame chemical vapor deposition (FF-CVD) method we developed. The cell efficiency approaches 4.2% with the film thickness about 9~ 13 mm if the film was used without further surface modification. As we known, one source of the energy loss in DSSC is charge recombination occurring between electrolyte and working electrode. In order to reduce charge recombination, we supply a thin metal oxide coating on TiO₂ electrode by using surface modification with metal-containing salt solution as precursor. A higher band gap of the metal oxide semiconductor provides energy barrier to prevent electron transfer back to the oxidative species, and then reduce the charge recombination. However, although thicker metal oxide coating may reduces the rate of charge recombination, it may also reduce the rate of electron injection into the conduction band of TiO₂ working electrode and reduce the cell efficiency. Therefore, the thickness of the metal oxide coating needs to be optimized for the DSSC efficiency enhancement. In this research, we choose ZnO as coating oxide, since ZnO has slightly higher band gap (3.37eV) than TiO₂ (3.2eV). We vary the concentration of zinc acetate solution from 0.1 M to 0.001 M for coating different thicknesses of zinc oxide on TiO₂ working electrode. The crystalline quality and morphologies of surface modified TiO₂ electrodes were characterized by using XRD and FESEM. EDS and XPS were used to confirm the presence of zinc on the surface of TiO₂ electrode. The coating thicknesses were determined by high resolution transmission electron microscopy (HRTEM). The DSSCs using these working electrodes were measured under AM 1.5G 100 mW/cm² by Keithley 2400 sourcemeter. As a result, as the concentration of zinc oxide precursor solution decreases from 0.1 M to 0.001 M, the cell efficiency increases from 1.9% to 5.6%. The short circuit current density ( ) increases from 3.59 mA/cm² to 11.89 mA/cm² as the concentration decreases from 0.1 M to 0.001 M . Furthermore, we also found that the excess thickness of zinc oxide coating will block the electron injection, and results in lower efficiency after surface modification. The drop of efficiency from 4.2% to 1.9% is mainly attributed to the decrease of from 10.08 mA/cm² to 3.59 mA/cm². It reveals that appropriate thin zinc oxide coating is necessary in reducing charge recombination, while maintaining the high rate of electron injection to the conduction band of TiO₂ working electrode.